I've been using a "IOGear USB 2.1 Bluetooth Micro Adapter GBU421" (
http://www.iogear.com/product/GBU421/) for about a two weeks now on my Mac Pro with it plugged into the back of my monitor. I did it because the signal strength from my Mac Pro to the Magic Mouse wasn't all that great (showed itself as "sluggish" or not tracking sometimes -- very annoying). No mouse problems since switching to it. It tracks all the time.
The only problems with this setup was that if I put the machine to sleep or reboot the USB is of course powered off so the mouse (and trackpad too) wouldn't always re-pair upon power-up. Sometimes I would need to re-pair the mouse, less so with the track pad.
A few days ago I did a "full factory reset" all the bluetooth stuff using the Bluetooth Explorer (part of the XCode developer tools) and now my mouse and trackpad are always paired after sleep and reboot. At least so far. As I said it's only been a few days since I did this.
I assume using a dongle on a macbook in place of the in-board bluetooth will work fine. Indeed these dongles are a small as they are specifically for use on portables.